1.Field of the Invention
The present invention relates to the bleaching of high-yield wood pulp with hydrogen peroxide in an alkaline medium.
By the term "high-yield wood pulp" ar intended the wood and paper pulps produced in a high yield by weight in the dry state relative to the starting lignocellulosic material, typically in the form of chips, constituting the dry state, namely, a yield greater than about 85% and often at least 90% by weight.
2. Description of the Prior Art
The aforedescribed wood pulps are characteristically produced by grinding the above starting material, typically wood chips, by means of a mill or a disc grinder, whether or not accompanied by a chemical and/or heat treatment.
Exemplary are those referred to in this art as mechanical, thermomechanical and chemithermomechanical pulps.
The bleaching of such pulps entails chemically decolorizing the chromophores of the wood constituents without solubilizing same. The chromophoric group responsible for the dark color of the pulp are principally borne by lignin and certain extracts (tannins).
Two types of processes are currently employed to carry out this decolorization:
(a) a first technique which entails reacting the pulp with a reducing agent, traditionally hydrosulfite, under mild conditions and in a neutral or slightly acidic medium. The reduction of the chemical groups responsible for the objectionable color results in a partial bleaching which is nonetheless sufficient for certain applications;
(b) a second technique which entails oxidizing the chromophores with hydrogen peroxide in an alkaline medium; the bleaching attained thereby is greater than in the first process, which explains why the peroxide process is presently increasingly employed to satisfy various paper quality requirements.
However, hydrogen peroxide is costly and its stability decreases as the pH increases. While it is accepted in this art that the pH range in which the bleaching with hydrogen peroxide is normally carried out extends from approximately 9 to approximately 11, as reported in "The Bleaching of Pulp", TAPPI Press, Singh ed., p. 227, Atlanta (1970), continuing need exists in the industry for further improvements in the area of such hydrogen peroxide processing.
Nonetheless, any such improvement must avoid the darkening of the pulp which is known to result from the presence of an alkaline agent, e.g., sodium hydroxide, NaOH ("alkaline" darkening), and the bleached pulp must be protected by acidification as described at page 229 of the TAPPI article indicated above.
It has thus been proposed to this art to use the hydrogen peroxide in at least two separate bleaching stages.
For example, in published application WO-84/02,366 a first stage is described in which the alkalinity conditions are abnormally high with a view to improving the mechanical quality of the fibers, and a second stage under conditions of normal alkalinity to compensate for the bleaching deficiency resulting from the first stage.
In TAPPI Journal, March 1987, pages 119 et seq., D. Lachenal also describes a two-stage process, but in which an amount of sodium hydroxide is used in the second stage which is much greater than that normally employed for a first stage.
Finally, French Patent No. 2,537,177 recommends maintaining conditions which are as uniform as possible throughout the bleaching operation while utilizing a succession of separate stages.
In each of the above processes, one stage differs from the following one in that the materials which have been used in the first, in particular hydrogen peroxide and alkaline agents such as sodium hydroxide, are intermediately removed from the pulp, at least for the most part, for example 90%, normally by washing and/or pressing the pulp. The resulting energy and capital costs militate against improving the efficiency of usage of the hydrogen peroxide.